Variable lift valve mechanisms for internal combustion engines are well known. In one example of such engines, a two-step roller finger follower rocker arm is known to pivot on a hydraulic lash adjuster (HLA) disposed on the engine block. An HLA generally comprises a slidable plunger that may be hydraulically extended to take up mechanical lash in a valve train. In an example where a valve lift change is accomplished by increasing oil pressure to the associated variable lift valve mechanism, the HLA is supplied with low pressure engine oil for conventional lubrication and lash adjustment. When a valve lift change is desired, oil pressure in the HLA is increased, and high-pressure oil flows through the same circuit in HLA to actuate the variable lift valve mechanism. To reverse the change the oil pressure is again reduced.
A problem exists in some prior art HLA assemblies having a single oil feed wherein the oil pressure is varied between the two modes. Because a minimum lash-adjusting oil pressure is present in the HLA at all times, the minimum required switching pressure must include the HLA minimum pressure. That is, the minimum required switching pressure must be higher than in other known systems wherein the lash adjuster and the switching element are independently supplied. Thus, providing dual independent oil supplies to a hydraulic lash adjuster represents an advance in the art.
Prior art HLAs receptive of such dual independent oil supplies comprise upper and lower plunger elements disposed within a body and defining a low pressure chamber or reservoir therebetween. The upper portion is dedicated to the auxiliary valve actuation function, and the lower portion to the hydraulic lash adjustment function. Because each of the elements is axially relatively short, the overall plunger may exhibit inferior side-load capability, resulting in relatively short working lifetimes and premature wear of such HLAs.
It is a principal object of the present invention to provide a dual feed hydraulic lash adjuster having superior side-load capability.